Wikipedia will not comply with any age checks required under the Online Safety Bill, its foundation says.
Rebecca MacKinnon, of the Wikimedia Foundation, which supports the website, says it would “violate our commitment to collect minimal data about readers and contributors”.
A senior figure in Wikimedia UK fears the site could be blocked as a result.
But the government says only services posing the highest risk to children will need age verification.
Wikipedia has millions of articles in hundreds of languages, written and edited entirely by thousands of volunteers around the world.
It is the eighth most-visited site in the UK, according to data from analytics company SimilarWeb.
The Online Safety Bill, currently before Parliament, places duties on tech firms to protect users from harmful or illegal content and is expected to come fully into force some time in 2024.
Neil Brown, a solicitor specialising in internet and telecoms law, says that under the bill, services likely to be accessed by children must have “proportionate systems and processes” designed to prevent them from encountering harmful content. That could include age verification.
Lucy Crompton-Reid, chief executive of Wikimedia UK, an independent charity affiliated with the foundation, warns some material on the site could trigger age verification.
“For example, educational text and images about sexuality could be misinterpreted as pornography,” she said.
But Ms MacKinnon wrote: “The Wikimedia Foundation will not be verifying the age of UK readers or contributors.”
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As well as requiring Wikipedia to gather data about its users, checking ages would also require a “drastic overhaul” to technical systems.
If a service does not comply with the bill, there can be serious consequences potentially including large fines, criminal sanctions for senior staff, or restricting access to a service in the UK.
Wikimedia UK fears the site could be blocked because of its stance.
It was “definitely possible that one of the most visited websites in the world – and a vital source of freely accessible knowledge and information for millions of people – won’t be accessible to UK readers (let alone UK-based contributors)”, wrote Ms Crompton-Reid.
There are currently 6.6 million articles on Wikipedia, and she said it was “impossible to imagine” how it would cope with checking content to comply with the bill.
She added: “Worldwide there are two edits per second across Wikipedia’s 300-plus languages.”
The foundation has previously said the bill would fundamentally change the way the site operated by forcing it to moderate articles rather than volunteers.
It wants the law to follow the EU Digital Services Act, which differentiates between centralised content moderation carried out by employees and the Wikipedia-style model by community volunteers.
On Tuesday, the House of Lords debated an amendment from Conservative peer Lord Moylan that would exempt services “provided for the public benefit”, such as encyclopaedias, from the bill.
Heritage Minister Lord Parkinson said he did not think this would be feasible, but added that Wikipedia was an example of how community moderation can be effective.
He said the bill did not say that every service needed to have age checks, and it was expected that “only services which pose the highest risk to children will use age verification technologies”.
Ms Crompton-Reid told the BBC that while Lord Parkinson’s remarks “reassured” her , the charity did not want to be relying on future goodwill and interpretation of legislation.
It would continue to urge that protections to community moderation were in the bill through measures such as an exception for public benefit websites like Wikipedia, she said.
A government spokesperson told the BBC the bill had been “designed to strike the balance between tackling harm without imposing unnecessary burdens on low-risk tech companies”.
Communications watchdog Ofcom will enforce it and would “focus on services where the risk of harm is highest”.
The government also believes it is unlikely Wikipedia would be classed as a category one service, those that would be subject to the bill’s strictest rules.